180 research outputs found
Star formation in galaxies along the Pisces-Cetus Supercluster filaments
We investigate the variation of current star formation in galaxies as a
function of distance along three supercluster filaments, each joining pairs of
rich clusters, in the Pisces-Cetus supercluster, which is part of the 2dFGRS.
We find that even though there is a steady decline in the rate of star
formation, as well as in the fraction of star forming galaxies, as one
approaches the core of a cluster at an extremity of such a filament, there is
an increased activity of star formation in a narrow distance range between
3-4/h_70 Mpc, which is 1.5-2 times the virial radius of the clusters involved.
This peak in star formation is seen to be entirely due to the dwarf galaxies
(-20<M_B<-17.5). The position of the peak does not seem to depend on the
velocity dispersion of the nearest cluster, undermining the importance of the
gravitational effect of the clusters involved. We find that this enhancement in
star formation occurs at the same place for galaxies which belong to groups
within these filaments, while group members elsewhere in the 2dFGRS do not show
this effect. We conclude that the most likely mechanism for this enhanced star
formation is galaxy-galaxy harassment, in the crowded infall region of rich
clusters at the extremities of filaments, which induces a burst of star
formation in galaxies, before they have been stripped of their gas in the
denser cores of clusters. The effects of strangulation in the cores of
clusters, as well as excess star formation in the infall regions along the
filaments, are more pronounced in dwarfs since they more vulnerable to the
effects of strangulation and harassment than giant galaxies.Comment: 10 pages, 9 figures; accepted for publication in MNRA
A Lens Mapping Algorithm for Weak Lensing
We develop an algorithm for the reconstruction of the two-dimensional mass
distribution of a gravitational lens from the observable distortion of
background galaxies. From the measured reduced shear, the lens mapping is
obtained, from which a mass distribution is derived. This is unlike other
methods where the convergence ("kappa") is directly obtained. We show that this
method works best for sub-critical lenses, but can be applied to a critical
lens away from the critical lines. For finite fields the usual mass-sheet
degeneracy is shown to exist in this method as well. We show that the algorithm
reproduces the mass distribution within acceptable limits when applied to
simulated noisy data.Comment: 4 pages, 6 figures, uses emulateapj5.sty (included); substantially
revised; a slightly shorter version (fewer figures) will appear in
Ap.J.Letter
Robust mixtures in the presence of measurement errors
We develop a mixture-based approach to robust density modeling and outlier
detection for experimental multivariate data that includes measurement error
information. Our model is designed to infer atypical measurements that are not
due to errors, aiming to retrieve potentially interesting peculiar objects.
Since exact inference is not possible in this model, we develop a
tree-structured variational EM solution. This compares favorably against a
fully factorial approximation scheme, approaching the accuracy of a
Markov-Chain-EM, while maintaining computational simplicity. We demonstrate the
benefits of including measurement errors in the model, in terms of improved
outlier detection rates in varying measurement uncertainty conditions. We then
use this approach in detecting peculiar quasars from an astrophysical survey,
given photometric measurements with errors.Comment: (Refereed) Proceedings of the 24-th Annual International Conference
on Machine Learning 2007 (ICML07), (Ed.) Z. Ghahramani. June 20-24, 2007,
Oregon State University, Corvallis, OR, USA, pp. 847-854; Omnipress. ISBN
978-1-59593-793-3; 8 pages, 6 figure
Finding Young Stellar Populations in Elliptical Galaxies from Independent Components of Optical Spectra
Elliptical galaxies are believed to consist of a single population of old
stars formed together at an early epoch in the Universe, yet recent analyses of
galaxy spectra seem to indicate the presence of significant younger populations
of stars in them. The detailed physical modelling of such populations is
computationally expensive, inhibiting the detailed analysis of the several
million galaxy spectra becoming available over the next few years. Here we
present a data mining application aimed at decomposing the spectra of
elliptical galaxies into several coeval stellar populations, without the use of
detailed physical models. This is achieved by performing a linear independent
basis transformation that essentially decouples the initial problem of joint
processing of a set of correlated spectral measurements into that of the
independent processing of a small set of prototypical spectra. Two methods are
investigated: (1) A fast projection approach is derived by exploiting the
correlation structure of neighboring wavelength bins within the spectral data.
(2) A factorisation method that takes advantage of the positivity of the
spectra is also investigated. The preliminary results show that typical
features observed in stellar population spectra of different evolutionary
histories can be convincingly disentangled by these methods, despite the
absence of input physics. The success of this basis transformation analysis in
recovering physically interpretable representations indicates that this
technique is a potentially powerful tool for astronomical data mining.Comment: 12 Pages, 7 figures; accepted in SIAM 2005 International Conference
on Data Mining, Newport Beach, CA, April 200
The observed distribution function of peculiar velocities of galaxies
We give the first determination of the observed peculiar velocity
distribution function for a representative sample of galaxies (within 50 Mpc
(H=100) of the Local Group), which includes a wide range of clustering
properties. We explore in detail the effects of uncertainties in sampling and
in distance measures on the estimated distribution function. The observed
distribution function is consistent with an earlier prediction of gravitational
clustering, over the entire range of peculiar velocities, from field galaxies
to rich clusters, on scales up to 50 Mpc (H=100). In the simplest consistent
model, most of the inhomogeneous mass of the Universe is in galaxies or their
halos. We estimate the ``Mach Number'' for the bulk flow within 50 Mpc (H=100)
from us to be approximately 0.8, which includes the effect of high-dispersion
galaxies in clusters.Comment: 28 pages, gzipped postscript file, including 11 figures and 2 tables;
To appear in the Astrophysical Journal, March 199
FLASH redshift survey - I. Observations and Catalogue
The FLAIR Shapley-Hydra (FLASH) redshift survey catalogue consists of 4613
galaxies brighter than \bJ = 16.7 (corrected for Galactic extinction) over a
605 sq. degree region of sky in the general direction of the Local Group
motion. The survey region is an approximately 60\degr \times 10\degr strip
spanning the sky from the Shapley Supercluster to the Hydra cluster, and
contains 3141 galaxies with measured redshifts. Designed to explore the effect
of the galaxy concentrations in this direction (in particular the Supergalactic
plane and the Shapley Supercluster) upon the Local Group motion, the 68%
completeness allows us to sample the large-scale structure better than similar
sparsely-sampled surveys. The survey region does not overlap with the areas
covered by ongoing wide-angle (Sloan or 2dF) complete redshift surveys. In this
paper, the first in a series, we describe the observation and data reduction
procedures, the analysis for the redshift errors and survey completeness, and
present the survey data.Comment: 12 pages, 12 figures, mn.sty, submitted to MNRA
Mapping the Physical Properties of Cosmic Hot Gas with Hyper-spectral Imaging
A novel inversion technique is proposed to compute parametric maps showing
the temperature, density and chemical composition of cosmic hot gas from X-ray
hyper-spectral images. The parameters are recovered by constructing a unique
non-linear mapping derived by combining a physics-based modelling of the X-ray
spectrum with the selection of optimal bandpass filters. Preliminary results
and analysis are presented.Comment: 6 pages, 5 figures; accepted by the 5th IEEE Workshop on Application
of Computer Vision (WACV/MOTION 2005), Breckenridge, CO, USA, 2005; uses
ieee.cls (included). For a pdf version with full-resolution figures, try
http://www.cs.bham.ac.uk/~exc/Research/Papers/ieee_astro_05.pd
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